The present invention relates to a method for producing a paper grade in which hydrophobizing paper sizes are used. These sizes have a reactive functional group capable of forming covalent bonds with cellulose fiber and such hydrophobic tails thereof that are directed outward from said fiber.
The present invention relates also to a method for producing a paper grade having additives in its furnish, in which method alkaline hydrophobizing paper sizes are used. Most fine paper grades are manufactured under alkaline conditions because of the facility of using precipitated calcium carbonate (PCC) as a filler. Said filler gives an increased durability against ageing, and better brightness. The water circulation of a papers machine has also been possible to close more complete.
Current printing applications of fine paper grades set a particular weight on sizing, examples of the latter being non-impact printing (NIP) and, particularly, ink-jet printing. Conventional office paper grades have not been able to meet the requirements set for so-called xe2x80x9cmulti-printable, or multi-purposexe2x80x9d office paper, i.e. suitable for use in varying types of copiers and printers including ink-jet printers.
According to experiences gathered from the results of ink-jet printing, printing quality is affected by the fiber composition, and thereby chiefly by the ratio of coniferous to deciduous wood. As to the quality of finished paper, the structure and topography of pores in the finished sheet are crucial to the outcome of the printing process in the discussed method. In terms of paper qualities, the printing result is determined by the noncompressible grain of the sheet and other parameters characterizing the ink absorption capability of the sheet. A paper grade optimized for ink-jet printing is required to have a sufficient capability of adsorbing the printing ink, yet permitting the ink to dry at a sufficiently fast rate before the ink can spread along the fibers or into the pores of the sheet structure. Thence, the surface-chemical interactions of the sheet with the ink are accentuated in ink-jet printing.
In addition to the basic factors related to the paper structure, the quality of ink-jet printing can be modified by means of additives used in papermaking such as hydrophobizing internal sizes and surface size formulations, surface size starches and pigments of high surface area.
Formulations for paper sizing have been developed in the art with the aim of modifying the surface-chemical properties of the paper and improving the black-and-white monochrome printing quality by virtue of increasing the hydrophobicity of the paper. By elevating the hydrophobicity of the paper, it has been possible to achieve a better printing result of black ink on the sheet through improved control of ink absorption under capillary forces into the sheet structure in both the lateral and the depth directions of the sheet. This approach has resulted in a sharply defined printing pattern and elimination of black ink spread (wicking) on the sheet.
The most commonly used sizing formulations suitable for fine paper, especially manufactured under alkaline conditions, are based on alkenylsuccinic acid anhydrides (ASA) and alkyl-ketene dimers (AKD). Both of these size types have a reactive functional group capable of forming a covalent bond with the cellulose fiber, as well as hydrophobic tails directed away from the fiber. The character and orientation of these hydrophobic tails make the fiber water-repellent. AKD and ASA sizes are dosed as an emulsion into the wet end of the paper machine and the sizing power is developed in the dryer section and the machine roll.
Commercial-grade alkylketene dimer sizes containing one xcex2-lactone ring are made by dimerization from two saturated straight-chain fatty acid chlorides; the most commonly used alkylketene dimer sizes being made from palmitic and/or stearic acid. Alkenyl succinic acid anhydrides, or ASA compounds, are obtained as the reaction products of long-chain olefins (C15-C20) with maleic acid anhydride.
With the goal of higher hydrophobicity of the paper in internal sizing of paper it has been necessary to use a higher dosing rate of ASA and AKD sizes in the paper machine, whereby the runnability of the machine has been deteriorated and different types of contamination problems in the process increased.
The approach of using a higher degree of hydrophobicity for controlling the printing behaviour of black ink does not, unfortunately, give an optimal result in multi-color printing. In fact, this method has been able to improve ink hold-out, with improved density as a result. In multi-color printing the application rates of inks are however higher than in black printing, which, together with the higher absorption has frequently caused a nuisance of insufficiently slow drying of printed color inks, resulting the spreading and mixing of superimposed colors on the printed sheet (known as color bleeding).
Consequently, different attempts have been made to improve the quality of multi-color ink-jet printing for instance by varying the amount of the size used in the internal sizing of the paper, and using fillers having higher surface-area, in order to control the behaviour of the printing colors. Surface sizing is also one possibility to affect the printability of paper.
Although different approaches have been proposed for the improvement of sheet absorption capability and a balanced degree of sufficient hydrophobicity for ink-jet printing, the field is still looking for alternative methods of manufacturing paper grades optimized for multicolor ink-jet printing.
Consequently, the main aspect of the invention is to provide a method of manufacturing paper of mono- and multi-color ink-jet printable grade by de-watering a paper web from fiber pulp slurry, the method containing a step of adding a 2-oxetanone based size to the pulp slurry, the size being manufactured from greater number than one of fatty acids having a main chain comprising 6 to 22 carbons linked to each other by saturated bonds, and of which acids at least one is an acid with branched chain.
A further aspect of the invention is to provide a method of manufacturing a paper of mono- and multi-color ink-jet printable grade from fiber pulp slurry into a paper web, the method containing a step of adding a size onto the paper web, wherein the size is a 2-oxetanone based size manufactured from greater number than one of fatty acids, the acids having a main chain comprising 6 to 22 carbons linked to each other by saturated bonds, and of which acids at least one is an acid with branched chain.
A still another aspect of the invention is to provide a method of manufacturing a paper of mono- and multi-color ink-jet printable grade by de-watering a paper web from fiber pulp slurry, the method containing a step of adding a 2-oxetanone size to the pulp slurry, in which size the fatty acid base consists of a greater number than one of fatty acids having a main chain comprising 6 to 22 carbons, the main chains of the acids dominantly being of thoroughly saturated type, but including in at least one of the acids a branching.
The use of sizes based on 2-oxetanone has been known for a long time in papermaking (e.g., refer to U.S. Pat. No. 2,627,477, and J. W. Davis, et. al.: A new sizing agent for paperxe2x80x94alkylketene dimers, Tappi 1956, Vol. 39, No. 1, but this litterature does not mention the use of 2-oxetanone produced from saturated fatty acids, of which at least one posses a branched carbon chain.
Analogously to conventional AKD sizes, the sizes used in the methods of the invention may be made starting from fatty acids, whereby it is essential that at last one of the fatty acids have a branched carbon chain, which chains, however, contains no double bonds. The length of the carbon chain in the starting material fatty acids may vary in the range from 6 to 22 carbons.
It has been found according to one aspect of the invention that particularly a mixture of branched-chain and linear-chain (e.g., with a ratio of 40/60 to 60/40) gives optimal qualities for a paper grade intended for ink-jet printing, especially balanced qualities as well as for mono- and multi-color printing. The paper has proven to serve also as a xe2x80x9cmulti-purposexe2x80x9d office paper (suitable for printing machines of another type).
In terms of papermaking, herein it must be pointed out that the amounts of size required in the novel method for attaining a desirable end result will be smaller than those needed in conjunction with conventional size formulations, thus alleviating the contamination and dirt adherence problems caused by sizes in the paper machine.
The invention also relates to a paper grade manufactured by treating with a size formulation based on 2-oxetanone manufactured from fatty acids of which at least one posses a branched carbon chain. The paper may contain mineral fillers, such as calcium carbonate, especially precipitated calcium carbonate (PCC), and alum.
Stable emulsions of the novel sizes can be made in the same manner as standard AKD emulsions.
The paper grade according to the present invention is generally sized so that at least 200 g, advantageously at least 600 g, and most advantageously at least 1 kg of size is added per ton of paper.
The paper grade according to the invention achieves a balanced compromise in the adsorption and hydrophobicity qualities of the paper so that a high-quality printing result is achieved with both black and color inks (that is, the benefits include minimal show-through, high printing density, no wicking, no bleeding, and minimal raggedness of the printed contours when printing with a black ink or color on color. Moreover, such a balanced printing result is achievable by virtue of the paper grade according to the invention without resorting to coating of the sheet, improvement of hydrophobicity by surface treatment or using a higher amount of surface size starch above normal addition rates.
Furthermore, the size formulations according to the invention make it possible to attain a desirable end result in ink-jet printing with a smaller amount of size dosing than is that required with conventional AKD sizes, whereby the problems of paper machine contamination and adherence of dirt and fuzz to rolls plaguing conventional AKD sizes can be avoided.
One type of size formulation according to the present invention is a 2-oxetanone size made starting from isostearic acid or a mixture of fatty acids advantageously containing at least 40% of isostearic acid or some other fatty acid with a branched carbon chain.